In general, vehicle manufacturers employ high-strength and light parts to improve fuel efficiency to the vehicles since stronger and lighter materials are increasingly used in the field of vehicle body to satisfy a current social demand.
In general, high tensile strength steel plates having at least 980 MPa are used in order to reduce weight of vehicle materials.
However, the high tensile strength steel plates have poor formability due to stiffness and dimensional variation generated by spring back after forming, such that additional post processes are required.
The formability of the high tensile strength steel plates can be improved by heating a portion of or the entire plates and softening them. The method of forming the softened portion by heating can be done using a press forming process such as warm forming, hot forming, and hot stamping. In general, ultra high tensile strength having 1500 MP or over can be formed.
Therefore, various material heaters have been developed for forming by the press forming process as mentioned above requiring less influence from the forming conditions in order to minimize the cycle time.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure, and therefore may contain information that does not form the prior art already known in this country to a person of ordinary skill in the art.